Make broker ingress/filter serving compatible

[yolocs]

Background
As part of #2301, we want to make it possible for broker to make use of ksvc for filter/ingress.

Problem
The current filter/ingress implementation has a couple of things are incompatible with serving.

• Filter/ingress uses the default 9090 as the metrics port. This is conflict with serving's queue proxy which also uses 9090 as the metrics port.
• Filter/ingress uses env POD_NAME to retrieve the pod name for metrics and uses objectFieldSelector to reference metadata.name in the deployment template. However, serving doesn't support objectFieldSelector.

Potential Solution

• Use a different metrics port, e.g. 9092
• Do we really need the POD_NAME?

Exit Criteria
The filter/ingress can be deployed as ksvc.

Time Estimate (optional):
1d

[yolocs]

@grac3gao

[yolocs]

There was an attempt to support certain downward API at knative/serving#4102.

[yolocs]

/label proposal/0.13